The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo3/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo3/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co. In situ X-ray absorption spectroscopy (XAS) reveals that these two catalysts possess similar Pt–Pt and Pt–Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The similar activity of the two catalysts is thus ascribed to their comparable strain, ligand, and particle size effects. Ex situ XAS performed on D-PtCo3/HSC under different voltage cycling stage shows that the continuous dissolution of Co leaves behind the NPs with a Pt-like structure after 30k cycles. The attenuated strain and/or ligand effects caused by Co dissolution are presumably counterbalanced by the particle size effects with particle growth, which likely accounts for the constant specific activity of the catalysts along with voltage cycling.

Dealloyed PtCo 3 and PtCu 3 catalysts supported on high surface area carbon (HSC), which were synthesized under different conditions, were tested as cathode electrodes in proton exchange membrane fuel cells. The dealloyed PtCu 3 / HSC gave higher initial oxygen reduction reaction (ORR) kinetic activity but much worse durability in a voltage cycling test. Detailed characterization was undertaken to develop insights toward the development of catalysts with both high activity and good durability. In situ X-ray absorption spectroscopy (XAS) analysis showed that dealloyed PtCu 3 / HSC exhibited stronger bulk Pt−Pt compressive strains and higher bulk d-band vacancies (attributed in part to a greater ligand effect induced by Pt−Cu bonding) than dealloyed PtCo 3 /HSC, factors which can be expected to correlate with the higher initial activity of dealloyed PtCu 3 /HSC. Annular dark field (ADF) imaging and electron energy loss spectroscopy (EELS) mapping demonstrated that a strong majority of metal nanoparticles in both dealloyed PtCu 3 /HSC and PtCo 3 /HSC have variants of core−shell structures. However, the most prevalent structure in the dealloyed PtCo 3 /HSC gave multiple dark spots in ADF images, approximately half of which were due to Co-rich alloy cores and half of which arose from voids or surface divots. In contrast, the ADF and EELS data for dealloyed PtCu 3 /HSC suggested the predominance of Pt shells surrounding single Cu-rich cores. Further work is needed to determine whether the contrast in durability between these catalysts arises from this observed structural difference, from the differences between the corrosion chemistry of Cu and Co, or from other factors not addressed in this initial comparison between two specific catalysts.

We have observed certain anomalies in computer fitting of data from positron annihilations in polymers. These suggest to us that some reported ortho-positronium (0-Ps) lifetimes and intensities in these polymers could be artifacts of the computer-fitting procedure. To evaluate this hypothesis, we have developed a computer simulation of experimental data, which can then be used to test the accuracy of the fitting program. The input to this simulation consists of the lifetimes and intensities of any number of positron populations (including para-positronium and free positron decays), plus the spectrometer resolution function, a contribution from annihilation in the positron source, and random background. The simulation uses the computer's random number generator to make the output spectrum resemble an actual experimental curve. The output spectrum is then used as input to the usual fitting program POSFIT, which determines the best-fitting values of lifetime and intensity for three positron lifetime components. When the shortest lifetime, T~, was fixed at the theoretical value of 120 ps, the values of the other lifetimes, 72 and 73, were found to be very close to the values in the simulated input. When the simulated input contained several 0-Ps lifetime components-ql, 7g,p, 73.9, etc.-the fitted (apparent) value of 7 3 (~3 ,~~~) was extremely close to the number-average input value (73). However, the fitted value for the total intensity of these components departed significantly from the total input intensity. The deviations increase drastically when the full width at half-maximum (fwhm) L 280 ps. Incorporating these new perceptions, we report investigations of the temperature dependence and aging behavior of free volume in glass and melt states for six polycarbonates of different Tg's. We have also evaluated chemical effects attributable to e+-and y-irradiation. In seeking a way to minimize effects of exposure to radiation, we have developed a new method for comparison of rejuvenated samples with well-aged material.

The characterization of proton exchange membrane fuel cell electrodes is essential for understanding the electrode performance. In this paper, mercury intrusion porosimetry and the nitrogen adsorption method were used to measure pore size distributions and porosities (ϵ) of various electrodes, which were made with either platinum supported on amorphous carbon (Pt/VA) or platinum supported on graphitized carbon (Pt/VG), and had ionomer‐to‐carbon weight ratios (I/C) of 0.5, 1.0, and 1.5. The oxygen effective diffusivity ($ D_{{\rm O}_2}^{\rm eff} $) in electrodes was measured as a function of relative humidity (RH) in an apparatus that was previously described [Z. Yu, R. N. Carter, J. Power Sources 195 (2010) 1079–1084]. The tortuosity of electrodes at the dry condition (80 °C and 0% RH) was then determined from the measured porosities and $ D_{{\rm O}_2}^{\rm eff} $. For a given catalyst, as the I/C ratio increased, it was found that the electrode's mean pore size, porosity, and $ D_{{\rm O}_2}^{\rm eff} $ all decreased, but the tortuosity increased. For a given I/C ratio, the Pt/VA electrode exhibited larger mean pore size, larger porosity, larger $ D_{{\rm O}_2}^{\rm eff} $, and smaller tortuosity compared with the Pt/VG electrode. The contrast between Pt/VA and Pt/VG electrodes with the same I/C ratio indicates different ionomer distribution on the catalyst surface.

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To truly understand the origin of the enhanced oxygen reduction reaction (ORR) activity of Pt-based electrocatalysts over pure Pt, it is essential to characterize the electrocatalysts under ORR operating conditions. Herein, we report a dealloyed Pt1Co1 NP catalyst with Pt mass activity ∼4 times higher than that of Pt/C and with superior stability in proton exchange membrane fuel cells (PEMFCs). Advanced microscopy and in situ spectroscopy are combined to show that this electrocatalyst possesses a unique ordered PtCo core–ultrathin with nonporous Pt shell structure, and the structure maintains the desired compositional profiles in the near-surface region even during the ORR in acid. Ab initio FEFF8 calculations based on experimentally derived structures reveal that the catalytic enhancements result from cooperative compressive-strain and ligand effects induced by the Co enrichment in the compact core and near subsurface.

New positron lifetime experiments have been carried out for Bisphenol A polycarbonate. The influence of unavoidable e+ irradiation on the lifetime and intensity of o-Ps annihilation was investigated. Our results obtained using a state of the art lifetime spectrometer (count rate 670 cps with a 30-rCi 22Na source, time resolution 260 ps fwhm for 22Na) are free from potential artifacts due to irradiation damage. Three lifetime components were computed from each spectrum. The temperature dependence of the o-Ps lifetime (~3 ) and intensity (13) were measured in the range between 20 and 150 "C for the as-received material and between -30 and +180 "C after annealing above Tg. We find that 7 3 is unaffected by annealing but that 13 is significantly larger for the annealed samples. In addition, the time dependence of ~3 and 1 3 due to physical aging in the glassy state was investigated at 30 and 90 "C, respectively. While 7 3 appeared constant for both temperatures, 1 3 decreased significantly with aging time. Anomalies in the absolute T and Z values derived from such fits and the temperature dependence of these values indicate that a distribution of T3 lifetimes must be present in the o-Ps decay. This is consistent with the presence of a distribution of hole sizes which would be expected from current models of free volume in amorphous polymers. Variations in the distribution, e.g. conversion of larger into smaller holes may result in changes of the apparent 13. Adopting a simple free volume definition, a comparison of the thermal expansivity of the vacancy fraction and of the macroscopic value yields a free volume fraction f of 8% at Tg = 150 O C .